It is common knowledge that nowadays needle phosphors as well as powder phosphors are used in the production of storage phosphors screens for computed radiography.
In the case of powder phosphors use is often made of divalent europium activated alkaline earth metal fluoro-halide phosphors. Barium-fluoro-bromide/iodide compounds are commonly used. Storage phosphors used in such screens can have, for e.g. the following composition Ba0.928Sr0.07Eu0.002F1.05Br0.80I0.15. In the case of powder phosphor screens, these compounds are coated as powders in a layer on a support.
Generally, a barium-fluoro-bromide (BaFBrI) phosphor is coated onto a support, e.g. made of polyester. In particular, a support made of polyethylene terephthalate (PET) with a thickness of approximately 250μ is often used as support. Such support is usually supplied in the form of a roll. This roll is de-rolled and a phosphor composition is then coated on the support. After, the coated support is dried and a protective layer is coated on the dried phosphor layer. The finished layers are then slit and cut to the usual commercial dimensions of phosphor screens.
Such commercially usual formats depend on the envisaged application; when referring to medical diagnosis applications, general radiography, mammography or dental applications are usually included. Apart from medical diagnosis, non-destructive testing of materials is another field of application where such screens may also be used.
In contrast to said powder phosphor screens, in the case of needle phosphor screens, the phosphor layer is formed on the surface of a carrier or support, made for e.g. aluminum, in the form of needles through a vapour deposition technique. In the later case, cesium-bromide compounds are often used.
In the case of phosphor screens using phosphor powder compounds incorporating iodine, in the course of its use, a yellowing of the screens occur.
A drawback occasioned by such yellowing is that such screen is characterized by an unpleasing appearance for the user; it also gives to said screen a weary appearance. A further and more technical disadvantage is that when the screen is read out in a digitizer, an uneven or stained image is detected. This is the main disadvantage, since it compromises use of the plate for (medical) diagnosis.
The probable cause of this phenomenon is that iodine, present for e.g. 15 mol %, is available for oxidation at the surface of the grain and as a result is converted to iodine gas. This oxidation is catalysed by ultraviolet (UV) irradiation, humidity and acid atmospheric conditions (pH<7). The vapour pressure of the so formed iodine gas is low, and as a result it remains in the phosphor screen and does not, or only very partially, escapes from said screen.
The visual result is a noticeable yellowing of the screen. As such, this phenomenon does not adversely affect the functioning of the phosphor during a radiographic exposure. The screen stores the radiographic ‘latent’ image just like before the yellowing phenomenon. However, during the subsequent phase, i.e. is during the reading out of the phosphor screen by the digitizer, the formed iodine will partly absorb the blue emission light of the phosphor. As a result, these yellow stains will also become visual on the digitally read-out image as stained areas with a lower signal strength. This is clearly unacceptable since the quality of the medical diagnosis is negatively influenced by this event.
Iodide (I−) is far easier oxidized compared to bromide (Br−) and certainly compared to fluoride (F−). For the later two ions the problem as described above does not occur when storage phosphor screens are used in conformity with usual working conditions.
The yellowing problem of the phosphor screens and the resulting local weakening of the emission light occurs not only in the case of phosphor screens used for general radiography (e.g. for exposure of the lungs or the thorax) but for special radiographic applications such as mammography as well. In case of using said phosphor screens for non-destructive testing this is also a disadvantage. However, this situation is a major issue when such phosphor screens are meant to be used in dental applications.
The reason for the latter phenomenon is as follows: storage phosphor screens for general or mammography applications are usually stored in a cassette and thus they are shielded from light. In contrary, usually storage screens for dental applications are neither stored in a cassette nor in a transparent housing. Therefore, no shield from light is provided in such cases.
Typically, storage screens for dental applications are for each new exposure put in a new plastic envelope. After exposure, the phosphor screen is taken out of the respective envelope for being read out. Then, if the phosphor screen is intended to be used again it is placed in a new envelope. In between such uses this phosphor screen is exposed to UV light and air humidity.
Epoxide-containing compounds are known for long. In particular, oxirane (ethyleneoxide) is well known for application in sterilisation of objects.
The general chemical formula of oxirane is C2H4O2 and the structural formula is as follows:

Treating objects with oxirane is a method known as such but only aiming sterilisation. This method is used for e.g. in hospital environment for the sterilisation of drapes, gowns and surgical instruments generally.
There are also publications that describe the sterilization stimulable of phosphor screens by using this method. An example of such publication is the article “An evaluation of microbiologic contamination on a phosphor plate system: is weekly gas sterilization enough?” Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod. 2010; 109: 457-462. This article describes how microbiological contamination of photostimulable phosphor screen is countered by gas-sterilization using ethylene-oxide.
To this end, each Friday all phosphor screens are sterilized by using a model 4XL Steri-Vac gas sterilization chamber, produced by 3M Health Care, St. Paul, Minn., USA.
A similar publication is the US2008/0085228 A1. This document describes how a digitizer for stimulable phosphor screens (a so-called digitizer) comprises a sterilization or disinfection unit that applies a sterilization or disinfection treatment to a photostimulable phosphor screen. As disinfectant treatment various possibilities are described, including a gas-treatment. The use of ethylene oxide as gas for such purpose is disclosed.
The aim of the later two disclosures however is quite distinct from the aim of the present invention since only disinfection of the phosphor screens is intended. Besides, the treatment herein described is superficial and takes place at a time that the yellowing problem has already occurred and cannot be solved any more. In fact, as soon as the gaseous iodide has been formed in the phosphor layer, the yellowing problem occurs and cannot be remedied any more by a later superficial treatment of the phosphor layer.
From document EP 0 234 385 the yellowing problem of phosphor screens is well known. The solution disclosed herein comprises the addition of a compound having an epoxy group to the phosphor screen. In particular, whenever said phosphor screen comprises an iodine phosphor composition, such as a divalent europium activated alkaline earth metal fluoro-halide phosphor. This compound may be added to the layer containing the phosphor. In this case, the epoxy containing compound can also act as binder for the phosphor. In an alternate arrangement, such compound may be added to a neighbouring layer of the phosphor layer.
According to this disclosure, these compounds are proposed to be added as liquid epoxides to the phosphor layer composition; throughout the specification disclosure is made of polymer coatings, but also the 1,2 epoxypropane compound is mentioned (C3H6O). This method however has many drawbacks:
Most of the examples cited in the text are directed to a resin, which reactivity is inherently limited. The reason for that is because resin molecules are by definition long molecules and thus less mobile, or have a limited diffusion velocity. This resin with reduced mobility must bridge the distance to the phosphor particle. However, in quite a number of cases this is impossible because the phosphor layer quickly dries after coating. Therefore, the resin molecules are in fact immobilised in the phosphor layer and as a result of this they become useless for the aim as set forth above. This phenomenon and resulting disadvantage is even stronger when the resin molecules are present in a layer neighbouring the phosphor layer instead of being present in the phosphor layer itself.
One should also take into account the thickness of the phosphor layer; In the case of general medical radiographic applications, such thickness is of approximately 250μ, in mammographic applications it is of approximately 150μ, whereas in the case of dental applications it is even less thick, of approximately 100μ.
Should the molecular weight of the epoxy-containing compounds decreases the above mentioned disadvantages also decrease. However, in this case another drawback arises, namely the evaporation of low-molecular weight epoxy containing compounds during the coating- and drying operations of the phosphor layer. Because of the evaporation such compounds do not have the time, and consequently the opportunity, to react with the iodine compounds at the surface of the phosphor particles.
As result, the gain occasioned by the use of more reactive low-molecular weight compounds is lost through the loss of such compounds of the phosphor mixture, due to the evaporation of such compounds from the coated phosphor layer.
The practical result of this is that the still present reactive iodide at the surface of the phosphor particle after coating and drying of the phosphor layer, cannot be ‘treated’ by the still present immobile epoxides in such layer. Eventually, this gives raise to yellowing phenomenon of the screens causing the problems as described above.
A second disadvantage of this method is that the presence of such extra component reduces the packing density of the phosphor screen; this packing density should be as high as possible in case of stimulable phosphor screens.
A third disadvantage of this method is that the presence of such extra component negatively influences the drying characteristics of the phosphor layer in production. This, in turn leads to screen structure and variations in packing density. Such kind of variations in the packing density is of primary concern in the production of powder phosphor screens.
Finally, there is still another important disadvantage linked to method disclosed in EP 0 234 385: working with such resins, which comprise toxic components raise a number of issues in practical working conditions, ex. from the point of view of safety of the installations, such as increasing the risk of explosion and, from the point of view of the operating staff increased health risk due to carcinogenic components. Moreover, the end product, the stimulable phosphor screen still contains a certain amount of such compounds.
For all of the above mentioned reasons the need to develop a production method devoid of many or all of the above mentioned problems still remains.